Wax crystallization process



Sept. 8, 1959 F. R. sHUMAN, JR

WAX CRYSTALLIZATION PROCESS Filed Jan. 2, 1957 IN V EN TOR. FRANK R. SHUMAN,J

i VIIIAVIIJ ATTORNEY Y United States Patenti]L WAX `CRYSTALLIZATION PROCESS Frank R. Shuman, Jr., Chester Springs, Pa., assignor to Sun Oil Company, Philadelphia, Pa., a corporation of New Jersey Application January 2, 1957, Serial No. 632,177

3 Claims. (Cl. 20S-37) This invention relates to a process for paraffin wax crystallization wherein a wax-bearing oil is mixed with a solvent, the solvent-oil mixture is passed through a chiller in which its temperature is lowered to a point at which the wax will crystallize, and more particularly to a process and apparatus in which scraping of the interior wall of the chiller by a rigid scraper blade, in order to remove wax adhering thereto, may be avoided, whereby to avoid the wear and maintenance problems inherent in the use of conventional scraped face Chillers.

In commercial processes for the production of paraiiin wax from wax-bearing oils, it has been the practice to mix the oil with a solvent which is a good solvent for oil, but not for wax, such as methyl ethyl ketone, methyl isobutyl ketone, ethylene dichloride, methyl isobutyl carbinol, and the like, either alone or in combination with an aromatic hydrocarbon such as benzene or toluene, or a saturate Y such as pentane. The wax-solvent mixture is then passed to a scraped-face chiller in which the temperature is reduced to a point at which wax will crystallize. Such scraped-'face chillers consist of concentric pipes, with the wax-solvent mixture passing through the center pipe and a cold heat exchange material passing through the outer pipe. As the wax-solvent mixture is cooled, wax tends to crystallize out and deposit itself on the walls of the pipe, since the pipe is always colder than the wax-oil mixture, and initial crystallization of the wax will always take place at the tube surface. The inner pipe is therefore customarily provided with rotatable Scrapers whose outer edges contact the inner wall of the center pipe, and which are adapted to continuously scrape the wax from the wall, since if this surface were not continuously scraped to remove wax therefrom, heat transfer from the cooling medium would be greatly reduced, due to the low thermal conductivity of wax, and efficiency of the process would drop olf rapidly.

Since the scraper blades are initially in contact with the walls of the c'enter tube, friction is always present, with attendant wear. If the scraper blades are fixed, eventually a gap will build up between the blades and the wall, to allow an insulating film of wax to build up on the wall. If the scraper blades are made of steel, and are of such a construction that they are free, to move outwardly under the influence' of springs or vcentrifugal force so that contact with the walls is maintained, wear on both walls and scraper blades uis"excessive,and frequently eccentric, so that the walls may be pierced after a very short time on stream. Wear on the walls may be minimized by making the scraper blades of a soft material, such as wood, but in this case, wear of the blades is quite rapid. In any event, sooner or later wear becomes so great as to necessitate taking the process off stream, so that the apparatus may be taken apart for replacement of the scraper blades or the chiller wall. This is expensive, not only on account of the cost of the blades, but also because of lost production during the time the process is olf stream.

It is an object of this invention to provide a process for removing wax from the walls of a chiller which ICC minimizes wear of the walls or scraper blades, and which permits operation of the chiller without shutdown for maintenance for indenite periods of time.

In accordance with the present invention, a vertical, externally cooled, tubular cooler is provided with an axially mounted rotatable impeller blade. The impeller blade extends longitudinally of the cooler throughout the length of `the externally cooled -section of the cooler, and approaches the walls of the cooler closely, but does not make contact therewith, so as to eliminate wear due to contact of the blade with the tube walls. A wax-bearing solvent-oil mixture carrying in suspension solid particles of somewhat greater specific gravity than the solvent-oil mixture isr fed to the upper part of the cooler and is passed downwardly therethrough. The solid particles are of a shape such that a plurality of sharp edges are presented such as a cube, piism, pyramid, or the like, or mixtures thereof and their smallest dimension is greaterv than the clearance between the impeller blade and the chiller walls when the impeller is rotated at relatively low speeds. If a high speed impeller is employed to give a highly turbulent action to the slurry, the particles may be of smaller size, since they will be thrown against the walls with suiiicient force to knock off the wax. The particles should be substantially softer than the material from which the chiller and impeller blade are made, so that these materials will not be abraded by the particles, and should be tough enough so that the particles are not easily broken and their sharp edges are not readily rounded by contact with the impeller blade or the chiller walls. Particularly suitable materials for this purpose are polyethylene, particularly the high density Zieglertype polyethylene, nylon, cellulose acetate, polytetrafluoroethylene, and like plastic materials which are highly resistant to mineral oils. As the suspension of particles in the solvent-oil mixture is passed down through the chiller, they will be thrown outwardly against the chiller walls by centrifugal force induced by rotation of the impeller, and will be pushed over the surface of the walls in scraping relation thereto by the impeller blade whereby to remove wax adhering to the walls with inconsequential wear of the impeller blades or walls.

A mixture or slurry of wax crystals, solvent, oil and particulate solids is removed from the bottom of the chiller and is passed to a disengager in which the particulate solids settle to the bottom. These are removed and recycled to the chiller to form the solid phase of the feed to the chiller, while the supernatant liquor, consisting of a slurry of wax crystals in oil and solvent, is removed and passed to a filter or centrifuge in which the wax is separated.

After a number of passages through the chiller, the sharpedges of the particulate solids may have been dulled or rounded to such an extent that their eiciency in scraping is impaired to an undesirable degree. In such an event, fresh particles may be added to the feed, while the old particles are discarded. By so proceeding, the ,scraping eiciency may be continuously maintained in the chiller for an indefinite period of time without any necessityafor a shutdown for lmaintenance .to replace worn scraper blades. i

ln order that those skilled in the art may more fully appreciate the nature of my invention and the means for carrying it out, it will be more particularly described in connection with the accompanying drawing, in which Fig. l is a vertical cross-sectional view of a cooler adapted for use in practicing the invention; Fig. 2 is a horizontal cross-sectional view of the apparatus of Fig. l, taken along line 2 2 of Fig. 1; and Fig. 3 is a schematic flow sheet of the process of the invention.

Referring now more particularly to Fig. l, it will be noted that the chiller indicated generally by 10, comprises a casing 11 fitted with a feed inlet line 12, solids inlet line 13, and outlet line 14. A concentric casing 15 surrounds casing 11, and is spaced apart therefrom to provide an annulus 16 through which refrigerant fluid may flow. Refrigerant inlet line 17 connects with annulus 16 near the bottom of casing 15, while spent refrigerant flows outwardly from annulus 16 through refrigerant outlet line 18. Shaft 19 passes through bushing 20 at the lower end of casing 11, and is received in bearing 21 at the upper end of casing 11. Shaft 19 is provided with driving means whereby it may be rotated, such driving means being illustrated in Fig. 3 as motor 22, and carries impeller blade 23, the outer edges of which approach the inner walls of casing 11, but do not make contact therewith, leaving a clearance 24, which in commercial practice would have a width of from about l@ inch to 1A inch, depending on the over-all size of the equipment. As may be observed in Fig. 2, the edges of impeller blade 23 are rounded, for a purpose which will be hereinafter explained.

In the operation of my invention, a wax-bearing oilsolvent mixture is admitted to chiller 10 through line 12, and a quantity of particulate solids is passed to chiller 10 through line 13, while impeller blade 23 is being rotated at a speed suicient to throw the solids toward the wall of the chiller. The solids, for example polyethylene cubes, preferably have a least dimension approximately double the size of clearance 24. That is, if clearance 2.4 is ls inch, the cubes should be about 1A inch on each side. The volume ratio of liquid to particulate solids should be from about 3:1 to about 10:1. Refrigerant uid is admitted to annulus 16 through line 17 at a temperature and at a rate sufficient to reduce the temperature of the feed to the level desired at the outlet 14 of chiller 10, for example -l F.

The oil-solvent mixture and the particulate solids pass downwardly through chiller 10, and are removed through line 14. In passing downwardly, the solids 25 are thrown outwardly against the wall of cooler 10, and tend to pile up between the walls and the impeller blade 23, as illustrated in Fig. 2. Rotation of the blade 23 will push the solids 25 along the walls of cooler 10 to remove, by their scraping action, the film of wax 26 which plates out on the walls. It will be noted that the rounded edges of impeller 23 minimizes any chopping of the solids, and also forces the solids into closer contact with the walls than is possible to bring about by centrifugal force alone.

Efuent from cooler 10, which consists of a solution of oil in solvent, wax crystals, the solid particles, is taken through line 14 to settler 27, in which the solid particles settle to the bottom. A suspension of wax crystals in the solvent-oil mixture is removed from settler 27 through line 28 for further processing to recover wax, while the solid particles are picked up by screw feeder 29 driven by motor 30, and are returned to the top of cooler via line 13V and valves 31 and 32.

After a number of passes through cooler 10, the solid particles may have their sharp edges worn suiciently that their scraping efficiency is impaired. When this happens, valves 31 and 32 are adjusted so that worn particles may be discharged through line 33, while new, sharpedged particles are added to the system through line 34'. When a suflicient amount of new particles has been added, valves 31 and 32 are adjusted to again permit circulation .4 of the particles through line 13. Alternatively, small amounts of particles may be continuously added and subtracted to maintain the entire supply of particles in equilibrium sharpness. The elliciency of scraping in chiller 10 may thus be kept up for indefinite periods of time without the necessity of shutting the process down and dismantling the equipment for replacement of worn parts.

I claim:

1. A wax crystallization process comprising passing a wax-bearing oil-solvent mixture into contact with a cooling surface in the presence of sharp-edged solid particles formed of a tough plastic material essentially insoluble in the oil-solvent mixture, substantially softer than the material from which the cooling surface is formed, and having a greater specific gravity than the oil-solvent mixture; and cooling the oil-solvent mixture to a wax crystallization temperature while moving the solid particles in scraping engagement with the cooling surface.

2. A wax crystallization process comprising continuously introducing a feed consisting of a wax-bearing oilsolvent mixture together with a plurality of sharp-edged solid particles having a greater specific gravity than the oil-solvent mixture to the interior of a tubular externally cooled chiller; cooling said oil-solvent mixture to a wax crystallizationv temperature while moving said solid particles by centrifugal force to the wall of the chiller; causing the solid particles to move in scraping engagement with the wall' of the chiller; continuously recovering solid particles together with oil-solvent bearing wax crystals in suspension; separating solid particles from the oil-solventwax suspension; and recycling the separated solid particles to the chiller in admixture with fresh feed, said solid particles being formed of a tough plastic material essentially insoluble in the oil-solvent mixture, and substantially softer than the materials of construction of the chiller.

3. A wax crystallization process which comprises continuously introducing a feed consisting of a wax-bearing oil-solvent mixture together with a plurality of sharpedged solid particles having a specific gravity greater than the oil-solvent mixture to the interior of a tubular externally cooled chiller having a rotatable 'nnpeller blade axially disposed therein, said impeller blade having a clearance between its outer edges and the interior wall of the chiller, said solid particles having a least dimension greater than said clearance; cooling the wax-solvent mixture to a wax crystallization temperature while rotating said impeller blade at a speed suiiiciently great to force the solid particles into contact with the wall of the chiller by centrifugal force; causing the solid particles to be pushed by the impeller blade in scraping engagement with the chiller wall; continuously recovering solid particles together with oil-solvent bearing wax crystals in suspension; separating solid particles from the major portion of the oil-solvent-wax suspension; and recycling the separated solid particles to the chiller in admixture with fresh feed, said solid particles being formed of a tough plastic material essentially insoluble in the oil-solvent mixture and substantially softer than the materials of construction of the chiller and impeller blade.

Referencesv Cited in the tile of this patent UNITED STATES PATENTS 2,584,966 Reeves Feb. 5, 1952 

1. A WAX CRYSTALLIZATION PROCESS COMPRISING PASSING A WAX-BEARING OIL-SOLVENT MIXTURE INTO CONTACT WITH A COOLING SURFACE IN THE PRESENCE OF SHAPR-EDGED SOLID PARTICLES FORMED OF A TOUGH PLASTIC MATERIAL ESSENTIALLY INSOLUBLE IN THE OIL-SOLVENT MIXTURE, SUBSTANTIALLY SOFTER THAN THE MATERIAL FROM WHICH THE COOLING SURFACE IS FORMED, AND HAVING A GREATER SPECIFIC GRAVITY THAN THE OIL-SOLVENT MIXTURE; AND COOLING THE OIL-SOLVENT MIXTURE TO A WAX CRYSTALLIZATION TEMPERATURE WHILE MOVING THE SOLID PARTICLES IN SCRAPING ENGAGEMENT WITH THE COOLING SURFACE. 